How should I practice Find Root IDs from Parent References?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Zip phone screen problems in FastPrep.

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Find Root IDs from Parent References

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Problem statement

Each row in nodes represents one JSON-like object as [id, parentId]. Every id is unique. An empty parentId represents null.

A node is a root when its parentId is empty or when that parent ID does not appear among the supplied node IDs. Return all root IDs in lexicographic order.

Function

findRootIds(nodes: String[][]) → String[]

Examples

Example 1

nodes = [["a",""],["b","a"],["c","a"],["d","missing"]]return = ["a","d"]

Node a has no parent, and d references a parent that is absent. Nodes b and c reference a.

Example 2

nodes = [["child","root"],["root",""]]return = ["root"]

Input order does not matter; root is the only node without a present parent.

Example 3

nodes = [["x","y"],["y","x"]]return = []

Both parent references are present, so this cycle contains no root under the stated rule.

Constraints

0 <= nodes.length <= 100000.
nodes[i].length == 2.
Each id is a distinct non-empty string.
Each parentId is either empty or a non-empty string.
The combined length of all strings is at most 10^6.